1. Field of the Invention
The present invention relates to a wet-type developing device employing a wet-type developer containing toner particles and carrier liquid, and a wet-type image forming apparatus including the wet-type developing device.
2. Description of the Related Art
In recent years, various types of wet-type image forming apparatuses have been proposed which employ a wet-type electrophotographic method to achieve printing of high-quality images on demand. In the wet-type electrophotographic method, image formation is performed using liquid toner (wet-type developer) containing carrier liquid and toner particles.
This type of wet-type image forming apparatus includes a wet-type developing device, in which an electrostatic latent image is developed by supplying a wet-type developer carried by the developer carrier to an image carrier having the electrostatic latent image formed thereon. The wet-type developer remaining on the developer carrier after the development is removed from the surface of the developer carrier for next development by a removing unit such as a cleaning blade.
Japanese Laid-Open Patent Publication No. 2006-030719 (document 1) is an exemplary document disclosing such a wet-type developing device configured to remove the wet-type developer remaining on the developer carrier.
The wet-type developing device disclosed in document 1 includes: the developer carrier for carrying the wet-type developer and developing the latent image formed on the image carrier; the removing unit for removing the wet-type developer remaining on the developer carrier after the development; and removal assisting means, disposed at an upstream side relative to the removing unit, for reducing cohesive force of the toner particles contained in the wet-type developer remaining on the developer carrier.
With such a configuration, the toner particles contained in the remaining wet-type developer after the development can be suppressed from being caught between the cleaning blade serving as the removing unit and the developer carrier, thereby removing the wet-type developer without damaging the surface of the developer carrier.
Meanwhile, although the configuration of removing the wet-type developer from the developer carrier is not disclosed, Japanese Laid-Open Patent Publication No. 2005-121816 (document 2) is an exemplary document disclosing a wet-type image forming apparatus configured such that a toner image formed by developing an electrostatic latent image using a wet-type developer is transferred to a transfer member and then remaining wet-type developer is removed from an image carrier after the transfer for the sake of next transfer.
The wet-type image forming apparatus disclosed in document 2 includes: the image carrier for carrying the toner image formed by supplying the wet-type developer containing toner particles and carrier liquid and for conveying the toner image to a transfer position; a removing unit, disposed at a downstream side relative to the transfer position, for removing remaining wet-type developer from the image carrier after the transfer; and a dispersing unit, disposed between the transfer position and a removal position for the removing unit, for dispersing the toner particles contained in the wet-type developer on the image carrier.
With such a configuration, the toner particles contained in the wet-type developer remaining on the image carrier can be suppressed from being firmly fixed to the image carrier, thereby suppressing the toner particles after the transfer from adversely affecting the image to result in deteriorated image quality.
Here, regarding the wet-type developing device disclosed in document 1, it is described that before the wet-type developer remaining on the developer carrier is removed using the removing unit, the removal assisting means performs one of the following operations: neutralization of charges in the toner layer; formation of an AC electric field; and application of external force mechanically to the toner layer.
Only with the neutralization of charges in the toner layer on the developer carrier by the removal assisting means before removing the toner particles using the removing unit, the charges in the toner layer are distributed in the form of two layers, thus failing to cancel the electrostatic binding force of the developer carrier on the toner layer.
Also, only with the generation of AC electric field by the removal assisting means instead of the neutralization of charges, the electrostatic binding force of the developer carrier on the toner layer cannot be canceled sufficiently.
Also, only with the application of mechanical external force to the toner particles by the removal assisting means instead of the neutralization of charges and the generation of AC electric field, the electrostatic binding force of the developer carrier on the toner layer cannot be canceled.
Accordingly, also in each of the cases, a part of the toner particles once separated from the developer carrier are adhered to the surface of the developer carrier again due to electrostatic force before being removed by the removing unit.
Hence, in the wet-type developing device disclosed in document 1, the electrostatic binding force cannot be canceled sufficiently, with the result that the toner particles cannot be dispersed sufficiently in the wet-type developer and the toner particles are accordingly accumulated on the removing unit, disadvantageously.
Document 2 discloses the configuration of removing the wet-type developer remaining on the image carrier, but it is difficult to directly apply this configuration to the removing unit for the developer carrier, which is different from the image carrier.
Even if this configuration is applied to the removing unit for the developer carrier, the electrostatic binding force of the developer carrier on the toner layer cannot be sufficiently canceled only with the generation of AC electric field by the dispersing unit. Moreover, when the AC electric field is generated while the charges held by the toner layer are not sufficiently canceled, the toner particles are adhered to the dispersing unit, with the result that a sufficient dispersing effect cannot be obtained.
Thus, even when the configuration of removing the wet-type developer from the image carrier as disclosed in document 2 can be applied to the configuration of removing the wet-type developer from the developer carrier, the electrostatic binding force of the developer carrier on the toner layer cannot be canceled sufficiently, with the result that the toner particles are accumulated on the removing unit, disadvantageously.
As described above, in the wet-type developing device, when removing the remaining wet-type developer from the developer carrier after the development using the cleaning blade serving as the removing unit, the toner particles may be accumulated at an abutment portion at which the cleaning blade is in abutment with the developer carrier.
The accumulation of toner particles takes place due to a difference between the viscosity of the wet-type developer located near the surface of the developer carrier and the viscosity of the wet-type developer located away from the surface of the developer carrier in the wet-type developer remaining on the developer carrier.
Specifically, in the wet-type developer located near the surface of the developer carrier, toner particles are aggregated to exist locally in the form of a layer near the surface of the developer carrier, so that the toner density of the wet-type developer becomes very high. This leads to a very high viscosity of the wet-type developer located near the surface of the developer carrier.
On the other hand, the wet-type developer located away from the surface of the developer carrier includes a large amount of carrier liquid, resulting in a low toner density of the wet-type developer. This leads to a low viscosity of the wet-type developer located away from the surface of the developer carrier.
The wet-type developer having the low viscosity flows over the wet-type developer having the high viscosity and is then drained. On the other hand, the wet-type developer having the high viscosity stays at the abutment portion of the cleaning blade, resulting in the accumulation of toner particles on the cleaning blade.
Once the toner particles are accumulated, more toner particles are accumulated due to the wet-type developer being continuously conveyed by the developer carrier, with the result that they cannot be recovered to a developer recovery tank.
The wet-type developer removed by the removing unit and recovered to the developer recovery tank is desirably conveyed to the developer tank again and used repeatedly for development for the purpose of cost reduction. However, when toner particles are accumulated, it becomes difficult to recover the toner particles and the accumulated toner particles need to be discarded during maintenance. This leads to increase in an amount of discarded toner particles, with the result that the toner particles cannot be efficiently reused, disadvantageously.
Moreover, when an excessive amount of toner particles are accumulated by repeatedly performing development, the accumulation of toner particles leads to contamination around the removing unit, resulting in deterioration of maintainability and image noise.